The present invention relates to a switching power supply comprising a DC-DC converter for supplying a stabilized DC voltage in an electronic equipment for industrial and consumer use, and relates, in particular, to a soft-switching power supply for soft switching such as zero-volt switching (ZVS).
A switching power supply converts an input DC voltage into an output of constant DC voltage, and is used in an electronic equipment such as a television set, a VTR and a personal computer. In a switching power supply, semiconductor devices, such as MOS-FET, IGBT and thyristor, are used as switches, whereby the ratio between the input and output voltages can be set through the duty ratio of turning ON and OFF. Accordingly, a switching power supply can stably output a predetermined DC voltage by controlling the turning ON and OFF. Since the electric power loss (switching loss) due to turning ON and OFF is generally small, a switching power supply is often used for the purpose of energy saving.
Reactance elements, such as transformer, inductor and capacitor, included in a switching power supply can be downsized and weight-reduced by turning ON and OFF at a higher frequency (switching frequency). On the other hand, the ratio between the input and output voltages of a switching power supply depends substantially only on the duty ratio of turning ON and OFF. Accordingly, a switching power supply can be rather easily downsized and weight-reduced with keeping its output voltage.
In recent years, there are rapidly increasing demands for energy saving, downsizing and weight reduction of various electronic equipments. Also regarding to switching power supplies, there are strong requirements for higher efficiency, smaller size, lighter weight and more stable output.
In order to meet such requirements, a higher switching frequency is necessary. However, a higher switching frequency causes a larger switching loss. Further, a part of the electric power dissipated as a switching loss causes surge current and voltage, which result in an adverse influence of noise on electronic equipments in the periphery.
Accordingly, the increase of switching frequency requires a switching technology for suppressing the switching loss. Known as such a technology is the soft switching. In soft switching, a switch in a transitional state turning from ON to OFF or vice versa is provided with a resonance voltage or current, whereby the switch turns from ON to OFF or vice versa when the voltage or current is at zero. In particular, zero-volt switching (ZVS) is the switching carried out when the voltage applied across the switch is at zero, whereas zero-current switching (ZCS) is the switching carried out when the current applied across the switch is at zero.
In accordance with soft switching, no electric power is applied across the switch at the instance of turning between ON and OFF. Accordingly, no switching loss in the electric power occurs in principle. In particular, in accordance with ZVS, no charge remains in the parasitic capacitance of the switch at the instance of turning ON. Therefore, no surge current occurs.
In a so-called isolation type switching power supply using a transformer for stopping a direct current between the power supply side and the output side, a prior art, such as disclosed in Japanese Laid-Open Patent Publication No. Hei 11-89232, is known, as a circuit for performing zero-volt switching using the energy stored in the transformer.
FIG. 15 shows a circuit constituting of a known switching power supply. The known example is of a full-bridge type converter, in which full-wave rectification is carried out in the secondary of a transformer 3.
FIG. 16 shows the pulse waveform of the current or voltage at each part indicated by an arrow in FIG. 15 of the known circuit.
As shown in FIG. 15, a switching control circuit 7 outputs switching signals G1, G2, G3 and G4 to four switching devices 11S, 12S, 13S and 14S, respectively. As shown in
FIG. 16, the switching signals G1, G2, G3 and G4 are rectangular waves having predetermined widths. The switching devices 11S, 12S, 13S and 14S are ON when the switching signals G1, G2, G3 and G4 are at a high potential (H), respectively, whereas the switching devices 11S, 12S, 13S and 14S are OFF when the switching signals G1, G2, G3 and G4 are at a low potential (L), respectively.
As shown in FIG. 16, the switching signal G1 changes from H to L at time T1, whereby the first switching device 11S turns OFF. Then, a resonance occurs among the leakage inductance of a primary winding 3a, a first capacitor 11C in a first switching section 11, and a second capacitor 12C in a second switching section 12. That is, a current I3 flowing through the primary winding 3a causes the first capacitor 11C to charge and the second capacitor 12C to discharge. Thus, the voltage V11 across the first switching device 11S increases from zero, while the voltage V12 across the second switching device 12S decreases from a maximum value Vin.
The voltage V11 across the first switching device 11S reaches the maximum value Vin, and, at the same time, the voltage V12 across the second switching device 12S reaches zero. Then, a second diode 12D connected to the second switching device 12S in parallel turns ON. At time T2 immediately after that, the switching control circuit 7 changes the switching signal G2 from L to H, thereby turning ON the second switching device 12S. In such a manner, ZVS is carried out for the turning ON of the second switching device 12S.
Similarly, a resonance occurs among the leakage inductance of the primary winding 3a, the first capacitor 11C and the second capacitor 12C during the interval from the time T7 when the second switching device 12S turns OFF to the time T8 when the first switching device 11S turns ON. After the voltage V11 across the first switching device 11S reaches zero, the first switching device 11S turns ON. In such a manner, ZVS is carried out for the turning ON of the first switching device 11S. Further, regarding to the turning ON of the third switching device 13S at time T4 and the turning ON of the fourth switching device 14S at time T6, ZVS is carried out similarly with a resonance among the leakage inductance of the primary winding 3a, a third capacitor 13C and a fourth capacitor 14C.
In addition to the above-mentioned full-bridge type converter of the prior art, switching power supplies with ZVS include a half-bridge type, a push-pull type and modifications thereof combined with an auxiliary winding, as disclosed in Japanese Laid-Open Patent Publication No. Hei 9-163740. In each of these, ZVS is carried out with, a resonance among the leakage inductance of the primary winding and the parasitic capacitors of the switches.
In a switching power supply disclosed in the Japanese Laid-Open Patent Publication No. Hei 9-163740, a bi-directional switching device is provided in parallel with a primary winding or an auxiliary winding. The bi-directional switching device comprises two switching devices interconnected in series. Each switching device is connected with a diode in parallel. The ends of the switching devices on the anode or the cathode side of the respective diodes are interconnected. The bi-directional switching device serves as a switching snubber (also called an active clamp). That is, the bi-directional switching device absorbs surge currents and voltages occurring when a switch for conducting electricity between the transformer and the input power supply is turned ON and OFF. Thus, the surge current and voltage are prevented from exerting the adverse influence of noise on other circuits in the periphery.
Recently, there is a growing number of the apparatus to be energized even in periods out of driving and the apparatus held on standby for a long time with being energized. The former apparatus include a personal computer and a facsimile machine, while the latter include a television set and a video tape recorder. In such an electronic equipment, the majority of the power is consumed during the standby. Accordingly, reduction of the standby power consumption is important in order to improve the energy saving in switching power supplies.
The current (load current) output from a switching power supply to a load is extremely low during the standby, in comparison with the driving. The load current during the standby is, in general, ⅕ to {fraction (1/10)} or less of that during the driving. Hereinafter, xe2x80x9ca heavy-load periodxe2x80x9d indicates a period when the load current is relatively large such as a period of the ordinary driving, whereas xe2x80x9ca light-load periodxe2x80x9d indicates a period when the load current is relatively small such as a period of the standby.
The known switching power supplies have the following problem in the light-load period. The switching loss of the above-mentioned prior art is certainly small in the heavy-load period, since the ZVS has been devised so as to be optimum in the heavy-load period. However, in the light-load period, the switching loss increases, since the ZVS cannot be carried out for the turning ON of the first switching device 11S and the third switching device 13S as follows.
In FIG. 16, the current I3 flowing through the primary winding 3a causes the third capacitor 13C to discharge when the fourth switching device 14S turns OFF at time T3. In heavy-load period, the load current is sufficiently large. Accordingly, the equivalent primary current thereof and the current I3 are sufficiently large. As a result, all of the charge stored in the third capacitor 13C can be moved against the input voltage Vin, thereby permitting the voltage V13 across the third switching device 13S to be at zero.
On the contrary, in the light-load period, the load current is small. Accordingly, the current I3 is small. As a result, all of the charge stored in the third capacitor 13C cannot be moved away during the resonance among the leakage inductance of the primary winding 3a, the third capacitor 13C and the fourth capacitor 14C. In this case, the switching loss increases since the voltage V13 across the third switching device 13S is not zero when the third switching device 13S turns ON at the same timing T4 as in the heavy-load period. In particular, the charge remaining on the third capacitor 13C moves vigorously at the timing of the turning ON of the third switching device 13S, thereby causing a surge current. Also when the first switching device 11S turns ON between times T7 and T8, the ZVS cannot be carried out in the light-load period, similarly to the case that the third switching device 13S turns ON. Therefore, in the light-load period, the switching loss increases and the surge current occurs.
An object of the present invention is to provide a switching power supply in which the ZVS reduces the switching loss in the light-load period and suppresses the occurrence of surge current and voltage, thereby realizing efficiency improvement and noise suppression.
In order to resolve the above-mentioned problem in a so-called full-bridge type switching power supply, a switching power supply according to the present invention comprises:
A) a DC-DC converter comprising:
a) four switching sections consisting of a first switching section, a seconds witching section, a third switching section, and a fourth switching section, each comprising 1) a switching device turned ON and OFF by switching signals from the outside, and 2) a diode and a capacitor each connected to said switching device in parallel;
b) a transformer comprising a primary winding and at least one secondary winding;
c) a rectifying circuit for performing full-wave rectification on the output of said transformer; and
d) a smoothing circuit for smoothing the output of said rectifying circuit; wherein
a) the end (cathode) of said first switching section on the cathode side of said diode and the cathode of said third switching section are connected to a high potential terminal of a substantially constant DC voltage source;
b) the cathode of said second switching section is connected to the end (anode) of said first switching section on the anode side of said diode, while the anode of said second switching section is connected to a low potential terminal of said substantially constant DC voltage source;
c) the cathode of said fourth witching section is connected to the a node of said third switching section, while the anode of said fourth switching section is connected to said low potential terminal of said substantially constant DC voltage source; and
d) one end of said primary winding of said transformer is connected to the junction point between said first switching section and said second switching section, while the other end of said primary winding is connected to the junction point between said third switching section and said fourth switching section;
B) a switching control section for outputting said switching signals to said switching devices at a predetermined switching frequency;
C) a load current sensing section for sensing the amount of load current output from said DC-DC converter; and
D) a delay section for delaying said switching signals of said switching control section for a predetermined delay time depending on said amount of load current sensed by said load current sensing section.
In a known full-bridge type DC-DC converter, the ZVS in the light-load period causes a problem, when the first switching section is turned ON after the turning OFF of the second switching section, and when the third switching section is turned ON after the turning OFF of the fourth switching section. However, in the above-mentioned full-bridge type switching power supply according to the present invention, the ZVS is carried out for the turning ON of the first switching section and the third switching section in the light-load period as follows.
The primary winding and the capacitors of the switching sections resonate in the primary of the transformer in the dead time between the turning OFF of the second switching section and the following turning ON of the first switching section, and the dead time between the turning OFF of the fourth switching section and the following turning ON of the third switching section. By virtue of the resonance, the current flowing through the primary winding decreases smoothly in the above-mentioned dead times.
On the other hand, the decreasing of the current flowing through the primary winding finally causes a substantial commutation in the secondary of the transformer, since full-wave rectification is carried out in the secondary of the transformer. In a center-tap type rectifier circuit, the transformer comprises two secondary windings interconnected in series, and the opposite ends of the secondary windings to the junction point thereof are connected to the respective rectifier diodes or the like. In the center-tap type rectifier circuit, the substantial commutation in the secondary indicates a transition from the state that a current flows through both of the secondary windings to the state that a current flows through only one of the secondary windings. In a bridge type rectifier circuit, the transformer comprises only one secondary winding, and both ends of the secondary winding are connected to a bridge consisting of four rectifier diodes or the like. In the bridge type rectifier circuit, the substantial commutation in the secondary indicates that two of the four rectifier diodes in the ON state are turned OFF. In the above-mentioned center-tap type and bridge type rectifier circuits, the rectifier diodes may be replaced by switching devices. In these rectifier circuits, full-wave rectification is carried out actively by controlling the switching devices.
In the light-load period, the ZVS cannot be carried out during the dead time of the same length as in the heavy-load period, since the current flowing through the primary winding is smaller than in the heavy-load period. However, if the dead time in the light-load period is longer than in the heavy-load period, the substantial commutation in the secondary can occur before the reversal of the current flowing through the primary winding. Then the primary winding""s inductance contributing to the resonance changes from the leakage inductance to the substantial whole of the self-inductance. This slows down notably the reduction in the current flowing through the primary winding. Accordingly, during a sufficiently long dead time in the light-load period, the capacitors can continue discharging for a time longer than in the heavy-load period.
According to the present invention, the delay section delays the timing of turning ON of the switching sections controlled by the switching control section for a predetermined delay time in comparison with the heavy-load period, when a state in the light-load period is recognized from the amount of load current sensed by the load current sensing section. Alternatively, the delay section delays the timing of turning OFF of the switching sections controlled by the switching control section for a predetermined delay time in comparison with the light-load period, when a state in the heavy-load period is recognized from the amount of load current sensed by the load current sensing section. Other timings of turning ON and OFF of the switching sections remain determined by a predetermined switching frequency. Accordingly, in both of the above-mentioned delays, the dead time in the light-load period can be longer than that in the heavy-load period. Thus, even in the light-load period, the voltage across the first switching section or the third switching section in the OFF state becomes zero during the dead time, whereby the ZVS is carried out similarly to the heavy-load period.
In order to resolve the above-mentioned problem in a so-called half-bridge type switching power supply, a switching power supply according to the present invention comprises:
A) a DC-DC converter comprising:
a) four switching sections consisting of a first switching section, a second switching section, a third switching section, and a fourth switching section, each comprising 1) a switching device turned ON and OFF by switching signals from the outside, and 2) a diode and a capacitor each connected to said switching device in parallel;
b) a first voltage dividing capacitor and a second voltage dividing capacitor interconnected in series;
c) a transformer comprising a primary winding and at least one secondary winding;
d) a rectifying circuit for performing full-wave rectification on the output of said transformer; and
e) a smoothing circuit for smoothing the output of said rectifying circuit; wherein
a) the opposite end of said first voltage dividing capacitor to the end thereof connected to said second voltage dividing capacitor is connected to a high potential terminal of a substantially constant DC voltage source;
b) the opposite end of said second voltage dividing capacitor to the end thereof connected to said first voltage dividing capacitor is connected to a low potential terminal of said substantially constant DC voltage source;
c) the cathode of said first switching section is connected to said high potential terminal of said substantially constant DC voltage source;
d) the cathode of said second switching section is connected to the anode of said first switching section, while the anode of said second switching section is connected to said low potential terminal of said substantially constant DC voltage source;
e) either the anodes or the cathodes of said third switching section and said fourth switching section are interconnected, while the opposite ends of said third switching section and said fourth switching section to the junction point thereof are connected to the respective ends of said primary winding of said transformer; and
f) one end of said primary winding is connected to the junction point between said first switching section and said second switching section, while the other end of said primary winding is connected to the junction point between said first voltage dividing capacitor and said second voltage dividing capacitor;
B) a switching control section for outputting said switching signals to said switching devices at a predetermined switching frequency;
C) a load current sensing section for sensing the amount of load current output from said DC-DC converter; and
D) a delay section for delaying said switching signals of said switching control section for a predetermined delay time depending on said amount of load current sensed by said load current sensing section.
In a known half-bridge type DC-DC converter, the ZVS in the light-load period causes a problem, when each of the first switching section and the second switching section directly connected to the constant DC voltage source is turned ON. However, in the above-mentioned half-bridge type switching power supply according to the present invention, the dead time in the light-load period can be longer than in the heavy-load period in a manner similar to the full-bridge type. Here, the dead time corresponds, in particular, to the interval when both of the first switching section and the fourth switching section are OFF in the time between the turning OFF of the fourth switching section and the following turning ON of the first switching section, and the interval when both of the second switching section and the third switching section are OFF in the time between the turning OFF of the third switching section and the following turning ON of the second switching section. The dead time in the light-load period is longer than in the heavy-load period according to the present invention. Accordingly, the capacitors can continue discharging in the light-load period for a time longer than in the heavy-load period. Therefore, the voltage across the switching section to be turned ON can become zero in the dead time, in spite of only a small current flowing through the primary winding in the light-load period. Thus, the ZVS is carried out similarly to the heavy-load period.
As another half-bridge type switching power supply, a switching power supply according to the present invention comprises:
A) a DC-DC converter comprising:
a) four switching sections consisting of a first switching section, a second switching section, a third switching section, and a fourth switching section, each comprising 1) a switching device turned ON and OFF by switching signals from the outside, and 2) a diode and a capacitor each connected to said switching device in parallel;
b) a first voltage dividing capacitor and a second voltage dividing capacitor interconnected in series;
c) a transformer comprising a primary winding, at least one secondary winding, and an auxiliary winding;
d) a rectifying circuit for performing full-wave rectification on the output of said transformer; and
e) a smoothing circuit for smoothing the output of said rectifying circuit; wherein
a) the opposite end of said first voltage dividing capacitor to the end thereof connected to said second voltage dividing capacitor is connected to a high potential terminal of a substantially constant DC voltage source;
b) the opposite end of said second voltage dividing capacitor to the end thereof connected to said first voltage dividing capacitor is connected to a low potential terminal of said substantially constant DC voltage source;
c) the cathode of said first switching section is connected to said high potential terminal of said substantially constant DC voltage source;
d) the cathode of said second switching section is connected to the anode of said first switching section, while the anode of said second switching section is connected to said low potential terminal of said substantially constant DC voltage source;
e) either the anodes or the cathodes of said third switching section and said fourth switching section are interconnected, the junction point thereof is connected to said low potential terminal of said substantially constant DC voltage source, and the opposite ends of said third switching section and said fourth switching section to said junction point thereof are connected to the respective ends of said auxiliary winding; and
f) one end of said primary winding is connected to the junction point between said first switching section and said second switching section, while the other end of said primary winding is connected to the junction point between said first voltage dividing capacitor and said second voltage dividing capacitor;
B) a switching control section for outputting said switching signals to said switching devices at a predetermined switching frequency;
C) a load current sensing section for sensing the amount of load current output from said DC-DC converter; and
D) a delay section for delaying said switching signals of said switching control section for a predetermined delay time depending on said amount of load current sensed by said load current sensing section.
Even in the switching power supply according to the present invention, that is, the half-bridge type converter further comprising the auxiliary winding, the dead time in the light-load period can be longer than in the heavy-load period in a manner similar to the above-mentioned half-bridge type. Therefore, according to the present invention, the voltage across the switching section to be turned ON can become zero in the dead time, in spite of only a small current flowing through the primary winding in the light-load period. Thus, the ZVS is carried out similarly to the heavy-load period.
In the above-mentioned full-bridge type and half-bridge type switching power supply, it is preferred that said delay time is substantially xc2xc of the resonance period determined by the self-inductance of said primary winding of said transformer.
In the heavy-load period, the leakage inductance of the primary winding contributes to the resonance among the primary winding and the capacitors of the switching sections in the dead time. In contrast, in the light-load period, the substantial whole of the self-inductance of the primary winding contributes to the resonance. Accordingly, by setting the above-mentioned delay time, the dead time in the light-load period can be easily set to be optimum for the discharging of capacitors. That is, the dead time in the light-load period is set to be an interval between the time when the resonance current starts to flow in the direction for discharging the capacitor and the time immediately before the resonance current reverses in the opposite direction for charging the capacitor.
In order to resolve the above-mentioned problem in a so-called push-pull type switching power supply, a switching power supply according to the present invention comprises:
A) a DC-DC converter comprising:
a) four switching sections consisting of a first switching section, a second switching section, a third switching section, and a fourth switching section, each comprising 1) a switching device turned ON and OFF by switching signals from the outside, and 2) a diode and a capacitor each connected to said switching device in parallel;
b) a transformer comprising a first primary winding and a second primary winding interconnected in series, and at least one secondary winding;
c) a rectifying circuit for performing full-wave rectification on the output of said transformer; and
d) a smoothing circuit for smoothing the out put of said rectifying circuit; wherein
a) the junction point between said first primary winding and said second primary winding is connected to a first potential terminal of a substantially constant DC voltage source;
b) one end of said first switching section is connected to a second potential terminal of said substantially constant DC voltage source, while the other end of said first switching section is connected to the opposite end of said first primary winding to the end thereof connected to said second primary winding;
c) the end of said second switching section on the same side as the connected end of said first switching section to said second potential terminal of said substantially constant DC voltage source is connected to said second potential terminal of said substantially constant DC voltage source, while the other end of said second switching section is connected to the opposite end of said second primary winding to the end thereof connected to said first primary winding; and
d) either the anodes or the cathodes of said third switching section and said fourth switching section are interconnected, and the opposite ends of said third switching section and said fourth switching section to the junction point thereof are connected to the respective ends of said first primary winding and said second primary winding opposite to the junction point thereof;
B) a switching control section for outputting said switching signals to said switching devices at a predetermined switching frequency;
C) a load current sensing section for sensing the amount of load current output from said DC-DC converter; and
D) a delay section for delaying said switching signals of said switching control section for a predetermined delay time depending on said amount of load current sensed by said load current sensing section.
Here, in case that the second potential terminal of the constant DC voltage source is the low potential terminal, both the anodes of the first switching section and the second switching section are connected to the second potential terminal. On the contrary, in case that the second potential terminal of the constant DC voltage source is the high potential terminal, both the cathodes of the first switching section and the second switching section are connected to the second potential terminal.
In a known push-pull type DC-DC converter, the ZVS in the light-load period causes a problem when each of the first switching section and the second switching section directly connected to the constant DC voltage source is turned ON. However, in the above-mentioned push-pull type switching power supply according to the present invention, the dead time in the light-load period can be longer than in the heavy-load period in a manner similar to the above-mentioned full-bridge type and half-bridge type. Here, the dead time corresponds, in particular, to the interval when both of the first switching section and the fourth switching section are OFF in the time between the turning OFF of the fourth switching section and the following turning ON of the first switching section, and the interval when both of the second switching section and the third switching section are OFF in the time between the turning OFF of the third switching section and the following turning ON of the second switching section. The dead time in the light-load period can be longer than in the heavy-load period according to the present invention. Accordingly, the capacitors can continue discharging in the light-load period for a time longer than in the heavy-load period. Therefore, the voltage across the switching section to be turned ON can become zero in the dead time, in spite of only a small current flowing through the primary winding in the light-load period. Thus, the ZVS is carried out similarly to the heavy-load period.
As another push-pull type switching power supply, a switching power supply according to the present invention comprises:
A) a DC-DC converter comprising:
a) four switching sections consisting of a first switching section, a second switching section, a third switching section, and a fourth switching section, each comprising 1) a switching device turned ON and OFF by switching signals from the outside, and 2) a diode and a capacitor each connected to said switching device in parallel;
b) a transformer comprising a first primary winding and a second primary winding interconnected in series, at least one secondary winding, and an auxiliary winding;
c) a rectifying circuit for performing full-wave rectification on the output of said transformer; and
d) a smoothing circuit for smoothing the output of said rectifying circuit; wherein
a) the junction point between said first primary winding and said second primary winding is connected to a first potential terminal of a substantially constant DC voltage source;
b) one end of said first switching section is connected to a second potential terminal of said substantially constant DC voltage source, while the other end of said first switching section is connected to the opposite end of said first primary winding to the end thereof connected to said second primary winding;
c) the end of said second switching section on the same side as the connected end of said first switching section to said second potential terminal of said substantially constant DC voltage source is connected to said second potential terminal of said substantially constant DC voltage source, while the other end of said second switching section is connected to the opposite end of said second primary winding to the end thereof connected to said first primary winding; and
d) either the anodes or the cathodes of said third switching section and said fourth switching section are interconnected, the junction point thereof is connected to said second potential terminal of said substantially constant DC voltage source, and the opposite ends of said third switching section and said fourth switching section to said junction point thereof are connected to the respective ends of said auxiliary winding;
B) a switching control section for outputting said switching signals to said switching devices at a predetermined switching frequency;
C) a load current sensing section for sensing the amount of load current output from said DC-DC converter; and
D) a delay section for delaying said switching signals of said switching control section for a predetermined delay time depending on said amount of load current sensed by said load current sensing section.
Even in the switching power supply according to the present invention, that is, the push-pull type converter further comprising the auxiliary winding, the dead time in the light-load period can be longer than in the heavy-load period in a manner similar to the above-mentioned push-pull type. Therefore, according to the present invention, the voltage across the switching section to be turned ON can become zero in the dead time, in spite of only a small current flowing through the primary winding in the light-load period. Thus, the ZVS is carried out similarly to the heavy-load period.
In the push-pull type converter with the auxiliary winding, similarly to the above-mentioned push-pull type converter, both the anodes of the first switching section and the second switching section are connected to the second potential terminal in case that the second potential terminal of the constant DC voltage source is the low potential terminal. On the contrary, both the cathodes of the first switching section and the second switching section are connected to the second potential terminal in case that the second potential terminal of the constant DC voltage source is the high potential terminal.
In the above-mentioned push-pull type switching power supply, it is preferred that said delay time is substantially xc2xc of the resonance period determined by the self-inductance of said primary winding and the self-inductance of said primary winding of said transformer.
In the heavy-load period, the leakage inductance of the primary winding contributes to the resonance among the primary winding and the capacitors of the switching sections in the dead time. In contrast, in the light-load period, the substantial whole of the self-inductance of the primary winding contributes to the resonance. Therefore, by setting the above-mentioned delay time, the dead time in the light-load period can be easily set to be optimum for the discharging of capacitors. That is, the dead time in the light-load period is set to be an interval between the time when the resonance current starts to flow in the direction for discharging the capacitor and the time immediately before the resonance current reverses in the opposite direction for charging the capacitor.
In each of the above-mentioned switching power supplies according to the present invention, said delay section delays said switching signals for turning ON said switching devices when said amount of load current sensed by said load current sensing section becomes substantially below a predetermined threshold value. Accordingly, in the heavy-load period, the switching signals from the switching control section are transferred to the switching. devices without passing through the delay section. Therefore, the switching signals are substantially free from the influence of noise and distortion in the heavy-load period. As a result, the precision of the switching control in the heavy-load period can be improved.
Further, in each of the above-mentioned switching power supplies according to the present invention, said load current sensing section may sense said amount of load current from any one of the currents of said switching devices, the current of said primary winding, the input and output currents of said DC-DC converter. The time average of the load current is reflected into the time average of the current flowing through each part of the DC-DC converter. Accordingly, the load current sensing section can sense the amount of load current from any one of the above-mentioned currents.
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.